Taggant materials have many applications including product identification, authentication, anti-counterfeiting, and encryption. Some known materials used as taggants include nanoparticles, birefringent materials, luminescent dyes, inks doped with magnetic or trace elements, holographic sheets, and radiofrequency identification (RFID) tags.
Although RFID tags can encode a significant amount of information, costs of these devices can be high, limiting their use to high value product tracking. They are also not suitable for covert tagging applications. Similarly, holographic images are used for high value product authentication and anti-counterfeiting. Again, price constraints limit the use of these materials. Birefringent material taggants are useful for product registration where a database of patterns has been established and where patterns are almost impossible to reproduce. However, these materials have a rotational and translational sensitive readout process which makes digital encoding or encryption virtually impossible.
Doped inks are standard inks that have been doped or impregnated with either an active material (such as magnetic particles) or with a trace element of a specific concentration. The first type of doped inks are usually absorptive in the visible spectrum and are not ideal for high security ink applications. On the other hand, the second type requires destructive testing techniques, limiting this application significantly.
Luminescent dyes are typically used for ultraviolet to visible wavelength conversion or for visible to near infrared wavelength conversion. However, they generally have inadequate absorption and emission properties. Different excitation sources are also necessary to access a mix of multiple luminescent molecules and the emission spectra are broad and asymmetric, making the retrieval of information difficult. Additional problems that may arise include possible interactions between two different luminescent molecules, immiscibility of multiple luminescent molecules in a common matrix or solvent, and the inherent instability associated with dye-based chromophores used under extreme environmental conditions.
Nanoparticle taggants typically use CdSe, CdS, PbSe, PbS or similar nanocrystalline quantum dot material. These materials emit either in the visible spectrum or in the near infrared spectrum upon excitation with an appropriate light source. While the level of encryption obtained with these materials can be extremely high, there are three major problems associated with use of these materials for security ink applications: 1) The materials are not solar blind and have an absorption in the visible spectrum, which ensures that these materials will emit under standing lighting conditions. This makes the inks generated using these materials conspicuous. 2) To hide these materials, black inks must be used to cover up the areas where these materials have been deposited. This in turn limits the applications where they may be used, especially in regards to covert applications or high-security applications, or even in areas where aesthetics of the graphic images or packaging may come into play. Use of Cd, Se, and Pb have adverse environmental consequences and limits the use of these elements to high value documents of a lasting nature.